Method of constructing insulated metal dome structure for a rocket motor

ABSTRACT

An elastomerized phenolic resin ablative insulation particularly suitable for use in connection with the thermal insulation of selected components of rocket motors. A composition for making the elastomerized ablative insulation is disclosed. Furthermore, an associated method of forming calendered sheets of material formed of the composition is disclosed. The preferred ingredients of the disclosed composition include acrylonitryle butadiene rubber, zinc borate, and phenol formaldehyde resin which can be cured and bonded to structures such as domes of open-ended rocket motors and other rocket motor components. The subject elastomerized ablative insulation is well suited for use independently or in connection with other insulative materials to form a thermal barrier which is highly resistant to the heat and the erosive nature associated with the combustion of propellant fuels, for example.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a divisional of application Ser. No.09/391,979, filed Sep. 8, 1999, pending.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to materials and methodsused to insulate structures from high temperatures and pressuresgenerated during the combustion of fuels. The present invention isparticularly suitable for insulating structures, including, but notlimited to, dome structures, nozzle structures, and igniter structuresof rocket motors, such as solid propellant rocket motors used in theaerospace industry.

[0004] 2. State of the Art

[0005] Solid propellant rocket motors have a center bore and/or cavityin the aft end of the motor in which combustion products of the solidpropellant flow and are directed through the throat of a nozzle.Combustion occurs on the surface of the propellant and the resultingcombustion products, upon passing through the throat, expand and areexpelled from the exit cone of the nozzle located at the aft-most end ofthe motor. Combustion products are accelerated from subsonic velocitiesat high pressure within the rocket motor to supersonic velocities atnear ambient pressure as the combustion products pass through the exitcone of the nozzle. The very high velocities at which the combustionproducts have been accelerated and directed by way of the rocket motorprovide the thrust needed to propel the craft, or spacecraft, to whichthe rocket motor or motors are mounted.

[0006] Open-ended solid propellant rocket motors typically have a muchlarger cavity in the aft end of the motor, referred to as the aft dome.The open-ended design is used to facilitate and ease the retraction ofmold tooling used in forming the internal geometry of the propellantgrain within the rocket motor. With open-ended rocket motors, combustionproducts can impinge directly on the aft dome at velocities exceeding300 feet per second (91 m/s) before exiting the nozzle.

[0007] Because of the high temperatures and pressures at whichpropellant fuels burn, typically of the magnitude of 5000° F. (2760° C.)and 1500 PSI (10,341 KPa), it is necessary to provide the internalsurface of such rocket motor domes, as well as other components andportions of the motor, with a thermally insulating material that canwithstand the impingement of high-velocity gases and oxidized, orpartially burned particles of fuel. The structure of the aft dome istypically made of aluminum, an alloy steel, or a fiber-resin compositeand would quickly rupture if directly exposed to the high-velocity,high-temperature combustion gases and oxidized particulates. Theinsulating material also serves to contain and protect the immediatelysurrounding area of the motor from the large amount of heat generated bythe rapid combustion of propellant fuels. Thus, the insulating materialmust not only be capable of withstanding the impact of high-velocitygases and particulates, which are very erosive to insulating materials,but must also be able to withstand being subjected to high temperaturesand high pressures upon the firing of the rocket motor.

[0008] Rocket motors have a nozzle exit cone, which directs the burninggas out of the motor and away from the craft. Such exit cones can be afixed-type cone, which is typically immovably mounted to the aft orrearward portion of the dome. Alternatively, and frequently, the exitcone can be a variable-angle or vectorable-type cone which is pivotablymounted to the aft portion of the dome so that the exit cone can bemoved angularly within a selected range to vector, or steer the craft inwhich the motor is installed, thus providing more directional control ofthe vehicle. Typically, the exit cone of a vectorable-type nozzle can bevectored within a range of 0 degrees to 10 degrees. The exit cone,whether a fixed-type or a vectorable-type, is typically attacheddirectly to the aft portion of the dome and is often canted at apreselected angle from the centerline, or longitudinal axis, of themotor. This is particularly true when the motor is configured as astrap-on booster rocket to provide increased launch capacity for aprimary or core space vehicle. A cant angle of up to 10 degrees from thecenterline, or longitudinal axis, of the motor is frequently used.However, other cant angles can be used as necessary. The cant is oftennecessary in crafts having multiple booster rockets, and is required todirect the exiting flame away from the centerline of the craft toprevent overheating or scorching of the craft itself or of adjacentlymounted motors.

[0009] Thus, the dome of an open-ended rocket motor as well as theinsulation contained within the interior of the dome must be configuredso as to allow the exit cone of the motor to be canted at a preselectedangle and/or vectorable within a preselected range of angles. The cantand/or sustained vector at a given angle gives rise to increased char inthe aft dome as gases are turned to exit through the nozzle throat. Theeffect of the nozzle cant also results in higher manufacturing costs dueto more complex machining, additional labor, and material scrap.

[0010] The art in the past utilized domes, typically made of apreselected metal alloy, in which two to three preformed rings oftape-wrapped carbon phenolic insulation were bonded into position in aconsecutive fashion within the dome to form a thermally insulatingbarrier therein. Tape-wrapped carbon phenolic insulation has been usedin the past to minimize inert weight due to increased thickness andbecause of its ability to withstand the mechanical and thermal erosionattributable to direct impingement of combustion products on theopen-ended aft dome. Each of these rings were usually manufacturedseparately because of the complex geometry of sequentially increasingdiameters in order to be fitted within the dome at a proper station. Thehollow dome likewise increases in diameter as viewed from the aftposition, or nozzle end, of the motor, and moving forward or away fromthe exit cone toward the dome where propellant fuel is undergoingcombustion.

[0011] In order to better understand and appreciate the presentinvention, reference is made to exemplary prior art insulators installedin the domes of open-ended solid propellant rocket motors as shown inFIGS. 1 and 2. FIG. 1 depicts an open-ended aft dome and nozzle of amotor having a fixed-type exit cone, whereas FIG. 2 depicts theopen-ended dome and nozzle of a motor having a vectorable-type exitcone.

[0012] More particularly, motor 2 depicted in FIG. 1 is provided with adome shell 10 which generally encases an open dome region 4 and a nozzlethroat region 6, and an exit cone shell 22 which generally encases anexit cone region 8. Dome shell 10 is typically a shell made of apre-selected metal alloy and includes a flange portion 26 for allowingthe dome shell 10 to be sealed and secured to the motor chamber case(not shown). Exit cone shell 22 is also typically made of a metal alloyand exit cone insulative liner 24 is typically made of a tape-wrappedcarbon fiber/phenolic composite material. The cant, or angle, at whichthe dome shell 10 must be configured in order to allow exit cone shell22 to extend away from the horizontal longitudinal centerline of themotor is designated as angle α. As mentioned previously, angle α canrange from 0 degrees to about 10 degrees, but a can be any suitableangle.

[0013] As can be seen in FIG. 1, nozzle throat region 6 is defined by anintegral throat entry (ITE) 18 which is typically formed of athree-directional or four-directional tape-wrapped carbon-carboncomposite material and is externally supported by a nozzle throatinsulator 20 typically formed of a unidirectional tape-wrapped carbonfiber/phenolic material.

[0014] It can further be seen in FIG. 1 that open dome region 4 isdefined by aft dome insulator 14 which lines the more aft portion ofdome shell 10 back to the integral throat entry 18. Forward domeinsulator 16 abuts with aft dome insulator 14 at joint interface 15 andinsulatively lines dome shell 10 from joint interface 15 forward toflange portion 26. Located behind dome insulators 14 and 16 and thusbetween dome insulators 14 and 16 and the interior surface of dome shell10 is a shear-ply layer 12. Shear-ply layer 12 is typically formed of anelastomeric material containing either silica powder or aramid fibers(e.g., fibers made of Kevlar® material) and a curable polymer such asethylene propylene diene monomer (EPDM), which is commercially availablefrom a number of sources. Shear-ply layer 12 provides a cushion betweenthe somewhat rigid dome insulators 14 and 16 and the inner surface ofdome shell 10 upon firing the rocket motor.

[0015] The construction of dome shell 10, shear-ply layer 12, aft domeinsulator 14, and forward dome insulator 16 is generally as follows.Shear-ply layer 12 is typically hand laid into essentially the fullinner surface of dome shell 10 by cutting and trimming calendered sheetsinto the proper size and configuration so as to conform to the innersurface of dome shell 10. A bonding system such as Chemlok® 205 primerand Chemlok® 236 adhesive available from the Lord Corp. is used toensure a proper bond between the shear-ply layer 12 and the innersurface of dome shell 10. Upon shear-ply layer 12 being properly laidand trimmed to fit the inner contour of dome shell 10, the shear-plylayer and dome shell are vacuum bagged and autoclaved to cure and bondshear-ply layer 12 onto the inner surface of dome shell 10. After beingcured, the exposed surface of shear-ply layer 12 is machined to a finalcontour and surface finish suitable for accommodating the insulatingmaterial to be bonded thereto.

[0016] Dome insulators 14 and 16 are made in accordance with previouslyknown materials and methods, and are first formed by laying down orwrapping a tape comprising carbonized rayon fibers that have beenimpregnated with, for example, a phenol formaldehyde resin about a workmandrel having a preselected contour and size which corresponds to theinner surface of the respective insulator. Such tape or tape wrapmaterial is commonly referred to within the art as tape-wrapped carbonphenolic composite material. Such tape-wrapped carbon phenolic materialis difficult to manufacture and is increasingly difficult to obtaincommercially due to prior sources ceasing to manufacture thesubcomponents of such material, or such sources no longer being inbusiness. Such precursor tape was originally manufactured by NorthAmerican Rayon Corp. (NARC), a fiber manufacturing subsidiary of NorthAmerican Rockwell Corp. in which the subsidiary is no longer inbusiness. The fiber tape upon having been carbonized and impregnatedwith phenolic resin (Fiberite product number MX 4926) sells for overUS$100.00 per pound (US$220/Kg), but there is no U.S. supplier currentlymanufacturing such tape. A multiyear supply of the carbon fiberprecursor was purchased and stockpiled by many companies when NARCannounced their plans to cease production of the precursor fiber.Alternatively, rayon fiber is available from CYDSA Corporation, aMexico-based supplier for which the mandatory qualifications andcertifications have not yet been fully conducted in order to be approvedas a certified vendor within the industry. However, once suitable andproperly qualified rayon fiber has been obtained, the rayon fiber mustfirst be woven into cloth or tape, and further must be carbonized by askilled, and often difficult to locate, carbonizing facility prior tobeing impregnated with a phenol formaldehyde resin. Such a resin isavailable either from Borden Corp. as part no. SC1-008 or AshlandChemical Corp. as part no. 91LD. One facility used in the past forimpregnating the carbon fiber with phenolic resin has been Fiberite Corpof Winona, Minn. However, as a practical matter, it is very difficult toorchestrate and ensure that such carbon resin composite material isproperly prepared to provide an end product of suitable quality.

[0017] Attempts at incorporating an alternate carbonized fiberprecursor, polyacrylonitrile (PAN), have been unsuccessful, in spite ofgood erosion resistance and adequate thermal performance. The PAN fiberswhen woven into tape and impregnated with phenolic resin suffer frominferior mechanical properties (i.e., low inter-laminar cross-ply andshear strength).

[0018] After the carbon fiber has been carbonized and impregnated with aphenolic resin, it is initially wrapped about respectively sizedmandrels to individually preform insulators 14 and 16. The mandrelscarrying respective tape-wrapped insulator preforms are vacuum baggedand the insulator preforms are then autoclave cured. Upon beingautoclave cured, or alternatively hydroclave cured, individually sizedand configured insulators 14 and 16 are removed from their respectivemandrels. Thereafter, the hollow, bowl-shaped insulators are machined inorder to configure the back, or outer surface, of each insulator to havea contoured matching dome shell 10 with shear-ply layer 12 previouslyinstalled therein. Additionally, the joint interfaces, such as jointinterfaces shown as 15 and 19 in FIG. 1, are machined so as to provide aproper surface for being cooperatively abutted against adjacentinsulators within dome shell 10. This machining process is quiteexpensive in terms of machining time, associated skilled labor, and theamount of material to be removed and hence simply scrapped. Thismachining expense is especially amplified with respect to theconstruction of aft insulator 14 which must be extensively machined inorder to provide the final contour needed to allow nozzle throat region6 and exit cone region 8 to be canted at a selected angle α. Uponexamining the lower portion of open dome region 4 of FIG. 1, it can beseen that the larger diameter insulator 16 is generally symmetricalabout the longitudinal centerline of the motor. However, smallerdiameter insulator 14 is asymmetrical about the longitudinal centerlinein order for the nozzle throat region 6 defined by integral throat entry18 and exit cone region 8 defined by exit cone shell 22 and exit coneinsulative liner 24 to extend downwardly to provide the required cantangle α. This asymmetrical configuration of insulator 14 requires thatan extensive amount of material be machined from the preform ofinsulator 14 because the preform may only be initially formed as asymmetrical workpiece in order for the resin-impregnated carbon fibertape to be properly laid down on the mandrel. In other words, insulator14 must first be made as a symmetrical, hollow bowl with the centermissing, then a large portion of the backside of the preformed bowl mustbe removed (ranging upwards of 50% of the original material) byexpensive and difficult multi-axis machining in order to provide theinsulator having the necessary cant or angled configuration to match theinterior of dome shell 10 and previously installed shear-ply layer 12.

[0019] Upon the respective joint interfaces and backsides of domeinsulators 14 and 16 having been machined, insulator 14 is firstinstalled and bonded into dome shell 10 against the inner facing surfaceof shear-ply layer 12 and longitudinally positioned against integralthroat entry 18. A structural epoxy adhesive, such as EA-934NA orEA-9394 available from Hysol-Dexter, Pittsburgh, Calif., is typicallyused for such bonding of insulator 14 to shear-ply layer 12 and at thebonding interface between the aft edge of insulator 14 and forward edgeof integral throat entry 18. Upon insulator 14 having been positionedinto place within dome shell 10, larger diameter insulator 16 is theninstalled and bonded to the remaining exposed portion of shear-ply layer12 and against the forward edge of aft insulator 14 to form a bondedjoint interface 15, also referred to as a secondary bond line, betweenthe two adjoining edges of insulators 14 and 16. Typically, the sameepoxy adhesive, such as the previously mentioned EA-934NA or EA-9394, isalso used as a bonding agent for the backside of bonding insulator 16against the inwardly facing surface of shear-ply layer 12 and at thejoint interface 15 between the aft edge of insulator 16 and the forwardedge of insulator 14. Dome insulators 14 and 16 are fully cured andbonded to shear-ply layer 12, which was previously bonded into domeshell 10. Lastly, the inner surfaces of dome insulators 14 and 16 aremachined to the final contour and surface finish which open dome region4 is to have. Thereafter, the completed dome assembly is ready to beinstalled as a major subcomponent of a rocket motor to which othercomponents can now be secured, including exit cone assembly 22/24.

[0020] Referring now to FIG. 2 of the drawings, illustrated is anopen-ended rocket motor 32 having a vectorable-type, or movable, exitcone. Motor 32 has a dome region 34 defined by dome shell 40 havingthree dome insulators, referred to as aft insulator 44, middle insulator46, and forward insulator 48. Motor 32 is further provided with anintegral throat entry 50 and a throat support insulator 52. Locatedabout the outer circumference of the throat region is a pivotingmechanism 58 which allows exit cone assembly 54/56 to be pivoted withina preselected range. As with motor 2, shown in FIG. 1, dome shell 40having a flange portion 60 is typically formed of a metal alloy and isconfigured to allow nozzle throat region 36 and exit cone region 38 tobe canted at a preselected angle α with respect to the longitudinalcenterline of motor 32.

[0021] Other than there being three dome insulators 44, 46 and 48positioned in an end-to-end consecutive manner, such as at jointinterfaces 45 and 47, and the three dome insulators 44, 46 and 48 bondedagainst the inner surface of shear-ply layer 42 bonded within dome shell40 of motor 32, as compared with only two dome insulators being bondedto a shear-ply layer 12 in the dome shell of motor 2, the previouslydiscussed materials and procedures for constructing and installingshear-ply layer 42 and dome insulators 44, 46, and 48 within dome shell40 are essentially the same as for shear-ply layer 12 and domeinsulators 14 and 16. As with asymmetrical aft dome insulator 14 of themotor shown in FIG. 1, asymmetrical aft dome insulator 44 of the motorshown in FIG. 2 must also have extensive multi-axis machining performedthereon in order for insulator 44 to be properly configured toaccommodate the cant angle of nozzle throat region 36 and exit coneregion 38. However, because three dome insulators are preferred, if notrequired, for motors designed to have a canted and/or vectorable-typeexit cone, the associated manufacturing, labor, and material scrap rateare thus increased proportionally.

[0022] In closed-ended rocket motors, the propellant fuel fills amajority of the aft dome cavity, thereby resulting in a “closed-ended”motor geometry as compared with an “open-ended” motor geometry aspreviously discussed and shown in FIGS. 1 and 2. In closed-ended rocketmotors, the propellant fuel is bonded directly to the insulationmaterial and/or an elastomeric liner, or stress-relief flap, which is inturn bonded to the insulation material. A material used in the past forinsulating chambers of rocket motors for forming internal insulation ofstructures incorporated in closed-ended rocket motors is set forth inTable 1-1 of an unclassified Department of the Navy data sheet entitledMolding Compound, Rubber, Butadiene Acrylonitrile, with Phenolic andBoric Acid, Compounding of, First Revision dated Oct. 13, 1966, and aschanged on Sep. 21, 1967. For convenience, such table is set forth inTable 1: TABLE 1 (Prior Art) Ingredients Parts by Weight (PBW) Hycar1051 (Butadiene Acrylonitrile 100 Elastomer) BKR 2620 (Phenolic Resin)120 Boric Acid (Powdered) 80 Stearic Acid 2 TMTD (Tetramethyl ThiuramDisulfide) 3 Zinc Oxide 5

[0023] Formulations for rubbers are typically called out in parts byweight (PBW). The vulcanizable rubber portion of the formulation (inthis case the Hycar 1051) is arbitrarily given a PBW of 100 and allother ingredients are called out as PBW in a level relative to the 100PBW of the vulcanizable rubber.

[0024] The above-listed ingredients needed to make the subject prior artmolding compound are available commercially and the process of combiningsuch ingredients is further set forth in the subject data sheet,incorporated herein by reference. Insulative material formed from theabove molding compound was specifically designed, tested, and approvedto be used as insulating material in closed-ended motors in which thesubject insulating material needed particular characteristics compatiblewith the propellant fuel to be bonded thereto. Furthermore, it is knownwithin the art that this molding compound has been used to form nozzlestationary shell insulators similar to insulator 182 in FIG. 6 and asindicated in Section II of an unclassified Department of the Navydocument of the Poseidon C3 Propulsion Test Program (Document No.SH050-A2A01HTJ, Report No. 10 dated Jul. 1, 1971). The prior artmaterial cited in Table 1 hereof was used as a stationary shellinsulator for the Poseidon Second Stage Motor design.

[0025] Materials originally designed, tested, and approved forclosed-ended motors, such as in Table 1 herein, have not generally beeninvestigated for use in open-ended motors due to differing designconstraints between closed-ended and open-ended motors. Whenincorporated in closed-ended rocket motor designs, the material in Table1, herein, was subjected to tightly controlled pre-ignition environmentsand limited to motor ignition temperatures greater than 70° F. (21° C.)because of concerns with strain capability at low temperatures.

[0026] Open-ended motor firing environments for this invention rangefrom 30° F. to 100° F. The severe thermo-mechanical erosive environmentstypical of the aft dome of an open-ended rocket motor result inincreased insulation thickness and, hence, additional inert weight, inorder to adequately protect the aft dome structure. For thoseelastomeric materials with superior erosion resistance, such as thematerial cited in Table 1 herein, problems with batch and productreproducibility and consistency, manufacturing defects (voids) andinsulator cracking due to aging were deemed higher risk options andunacceptable for an open-ended rocket motor production program.

[0027] The following documents are exemplary of rocket motor insulatorsknown within the art:

[0028] U.S. Pat. No. 4,492,779, issued to Junior et al. and entitledAramid Polymer And Powder Filler Reinforced Elastomeric Composition ForUse As A Rocket Motor Insulation, is directed to a process forinsulating solid propellant rocket motors with a composition comprisingaramid fibers, a powder filler, and vulcanizable elastomericcomposition;

[0029] U.S. Pat. No. 4,600,732, issued to Junior et al. and entitledPolybenzimidazole Polymer And Powder Filler Reinforced ElastomericComposition For Use As A Rocket Motor Insulation, is directed to anelastomeric composition comprising polybenzimidazole polymer fibers, apowder filler and a vulcanizable elastomeric composition;

[0030] U.S. Pat. No. 4,458,595, issued to Gerrish Jr. et al. andentitled Ablative Liner, is directed to an end-burning rocket motorhaving a first layer of silicone rubber and a second layer of anablative lining placed between the rocket motor casing and thepropellant grain; and

[0031] U.S. Pat. No. 4,956,397, issued to Rogowaski et al. entitledInsulating Liner For Solid Rocket Motor Containing VulcanizableElastomer And A Bond Promoter Which Is A Novolac Epoxy Or A ResoleTreated Cellulose, is directed to an insulating liner for a solid rocketmotor having a vulcanizable elastomeric composition, powder filler, anda cellulosic bond promotor.

[0032] Additionally, the inventors of the present invention are aware ofthe following U.S. patents:

[0033] U.S. Pat. No. 5,352,212, issued to Guillot and entitled Method ofInsulating a Rocket Motor, is directed to compositions of insulationscontaining thermoplastic liquid crystal polymers, fibers, andparticulate fillers;

[0034] U.S. Pat. No. 5,399,599, issued to Guillot and entitledThermoplastic Elastomeric Internal Insulation for Rocket Motors for LowTemperature Applications, is directed to compositions of insulationscontaining thermoplastic elastomers, an inorganic phosphorus compound, apolyhydric alcohol, a silicone resin, and chopped fibers;

[0035] U.S. Pat. No. 5,498,649, issued to Guillot and entitled LowDensity Thermoplastic Elastomeric Insulation for Rocket Motors, isdirected to compositions of insulations containing thermoplasticelastomers, a maleic anhydride modified EPDM, and carefully selectedfillers and chopped fibers; and

[0036] U.S. Pat. No. 5,762,746, issued to Hartwell, et al. and entitledMethod of Internally Insulating a Propellant Combustion Chamber, isdirected to compositions containing polyphosazene polymer and organicfiber filler.

[0037] Thus, it can be appreciated that there is a need within the artfor an insulating material having ingredients that can be readily andeconomically obtained and that can be readily formed and bonded intoselected structures of rocket motors more efficiently and more costeffectively, as compared to previously known materials and methods.

[0038] It can further be appreciated that there is a need within the artfor an insulating material that can withstand the high temperatures,high pressures and high particulate velocities encountered in theburning of propellant fuels in rocket motors without a substantialburden in increased inert weight to protect dome structures and adjacentmotor components and the craft.

[0039] Another need within the art is the ability to constructinsulators which are to be positioned within selected structures andcomponents of rocket motors, particularly those of asymmetricconfiguration, while minimizing the number and complexity of stepsrequired to preform such insulators. Furthermore, there is a need tominimize the amount of scrap or wasted material in constructing suchinsulators as well as to minimize the amount of difficult and expensivemulti-axis machining needed to construct such insulators.

[0040] There is yet a further need within the art for an insulatingmaterial having particular ablative qualities as well as having acoefficient of thermal expansion and strain modulus particularlysuitable for thermally insulating certain components of open-endedrocket motors and which can be obtained at a significantly lower cost ascompared to previously known insulative materials.

[0041] Another need within the art is for an insulating material thathas favorable aging characteristics, i.e., once an insulator isconstructed, it may be several years before the rocket motor in whichthe insulator is incorporated is actually fired. Thus the art wouldbenefit from having insulating materials having improved agingcharacteristics.

[0042] Yet another need within the art is for an effective insulatingmaterial in which the compound forming such insulating material iseasier to mix, has uniformly dispersed ingredients, and repeatedlyprovides insulative products of consistently high quality.

BRIEF SUMMARY OF THE INVENTION

[0043] The present invention provides a method of insulating a structureof a rocket motor having at least one surface on which an ablativeinsulative material is to be disposed. The method includes disposing atleast one layer of curable ablative insulative material on at least onesurface of the selected structure. The insulative material is generallyformed of a vulcanizable rubber, a flame retardant such as zinc borate,a phenolic resin and a cure system constituent, and may optionally havereinforcing fibers therein. The method preferably includes theinsulative material being formed of a compound including, but notlimited to, the following ingredients: acrylonitrile butadiene rubber,zinc borate, phenol formaldehyde resin, zinc oxide, tetramethyl thiuramdisulfide, and stearic acid. The insulative material may optionally beprovided with supportive or reinforcing fibers or fibrous elements suchas aramid, cotton (cellulose), sisal, polybenzamidazole, mineral wool,nylon, polyester, or carbon fibers. The method also includes curing atleast one layer of curable ablative insulative material that has beendisposed on at least one surface.

[0044] The present invention additionally provides a composition for anablative insulative elastomerized phenolic resin material generallycomprising: vulcanizable rubber, a flame retardant such as zinc borate,a phenolic resin and a cure system constituent, and which may optionallyhave reinforcing fibers therein. Preferably, the composition comprisesthe following ingredients: acrylonitrile butadiene rubber; zinc borate;phenol formaldehyde resin; zinc oxide; tetramethyl thiuram disulfide;and stearic acid. Preferably, the individual ingredients have thefollowing maximum parts by weight: acrylonitrile butadiene rubber—100;zinc borate—80; phenol formaldehyde resin—120; zinc oxide—5; tetramethylthiuram disulfide—3; and stearic acid—2. Preferably, a stoichiometricmaster batch is provided that consists of a mechanically ground andscreened phenol formaldehyde resin and zinc borate wherein the zincborate coats the ground resin and acts as a partitioning agent toprevent the ground resin from agglomerating into larger, nondispersableparticles or clumps. The stoichiometric master batch offers enhancedrepeatability and quality control in producing insulative materials fromthe disclosed composition by ensuring that the largest undispersed resinparticle does not exceed 100 mesh in size.

[0045] The present invention further provides a method of constructingan internally insulated metal dome structure of an open-ended rocketmotor. The method includes disposing a shear-ply layer formed of apreselected curable elastomeric material onto at least a portion of theinner surface of a dome structure and curing and bonding the shear-plylayer to a selected portion of the inner surface of the dome structure.The method further includes preforming at least one first dome insulatorabout a mandrel. This first dome insulator (or insulators) is preferablymade of a carbon phenolic composite material comprising carbonizedfibers impregnated with a preselected curable resin and generally has anouter surface and an inner surface. The first insulator is thenprecured. A carbon phenolic or other highly erosion-resistant andstructurally stable materials is required adjacent to the integralthroat entry (ITE) in order to ensure a smooth transition of combustiongases flowing into the nozzle throat region. The method further includesmachining at least a portion of the first insulator to a finalconfiguration and positioning and bonding the first insulator onto aselected portion of the inner surface of the shear-ply layer previouslydisposed and bonded onto the selected portion of the inner surface ofthe structure. The method yet further includes disposing at least onesecond dome insulator (or insulators) onto at least a portion of theinner surface of the dome structure longitudinally proximate to thefirst dome insulator, the second dome insulator being generally formedof a vulcanizable rubber, a flame retardant such as zinc borate, aphenolic resin and a cure system constituent, and which optionally maycontain reinforcing fibers therein. Preferably, the second domeinsulator is formed of a material, formed of a composition comprising atleast acrylonitrile butadiene rubber, zinc borate, a curable resin,tetramethyl thiuram disulfide, and stearic acid. Preferably, thecomposition is prepared by a mixing process incorporating astoichiometric master batch of resin and zinc borate. The second domeinsulator is then cured and bonded within the dome structure.Alternatively, the second dome insulator may be precured, machined to afinal contour and bonded to the dome structure using an epoxy adhesive.

[0046] The present invention also provides a thermal barrier forthermally insulating a structure. The thermal barrier is positioned toinsulate at least a predetermined portion of the structure and includesan ablative insulative material formed generally of a vulcanizablerubber, a flame retardant such as zinc borate, a phenolic resin and acure system constituent, and which optionally may contain reinforcingfibers therein. Preferably, the ablative insulative material is formedof a curable compound including the following ingredients: acrylonitrilebutadiene rubber, zinc borate, phenol formaldehyde resin, zinc oxide,tetramethyl thiuram disulfide, and stearic acid. The thermal barrier mayalso include a shear-ply layer formed of an elastomeric materialpositioned between the structure and a second insulative material formedof a fibrous material impregnated with a phenolic resin, wherein aportion of the ablative insulative material and a portion of the secondinsulative material abut against each other to form a secondary bondline therebetween.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0047]FIG. 1 is a cross-sectional view of a prior art open-ended rocketmotor having an insulated dome and a fixed-type exit cone;

[0048]FIG. 2 is a cross-sectional view of a prior art open-ended rocketmotor having an insulated dome and a vectorable-type exit cone;

[0049]FIG. 3 is a cross-sectional view of an open-ended rocket motorhaving a fixed-type exit cone and an insulated dome embodying thepresent invention;

[0050]FIG. 4 is a cross-sectional view of an open-ended rocket motorhaving a vectorable-type exit cone and an insulated dome embodying thepresent invention;

[0051]FIG. 5 is a cross-sectional view of an insulated igniter pelletcup portion of a rocket motor embodying the present invention; and

[0052]FIG. 6 is a cross-sectional view of an insulated nozzle of arocket motor having a stationary steel shell embodying the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0053]FIGS. 3 and 4 of the drawings depict exemplary open-ended domerocket motors incorporating insulation material embodying the presentinvention and installed within the respective dome structures, orshells, of each motor in accordance with the present invention. Motor72, shown in FIG. 3, is provided with a fixed-type exit cone whereasmotor 102, shown in FIG. 4, is provided with a vectorable-type exitcone. The exit cones, and thus the nozzles and dome shells of bothmotors, are canted at a preselected angle α from the longitudinalcenterline of the motor. In accordance with the reasons discussedpreviously, this cant angle can vary from 0° to about 10°.

[0054] In reference to FIG. 3, motor 72 generally encases and definesinternal dome region 74, nozzle throat region 76, and internal exit coneregion 78. A dome shell, or structure 80, having a mounting flange 96,is provided with a shear-ply layer 82, an aft insulator 84, and aforward insulator 86 that are bonded to the inner surface of dome shell80, which defines internal dome region 74. The remainder of motor 72,including nozzle throat region 76 being defined by a three-directionalor four-directional woven carbon-carbon composite integral throat entry(ITE) 88, a unidirectional tape-wrapped carbon phenolic throat insulatorsupport 90, a metal alloy exit cone shell 92 and a carbon phenoliccomposite exit cone liner or insulator 94 may be constructed in the samemanner and of the same materials as discussed previously with respect tothe exemplary prior art motor shown in FIG. 1.

[0055] Motor 102, shown in FIG. 4, similarly encases internal domeregion 104, nozzle throat region 106, and exit cone region 108. A metalalloy dome shell 110, having a mounting flange 130, is provided with ashear-ply layer 112, an aft insulator 114, and a forward insulator 118which are bonded directly or indirectly to the inner surface 111 of domeshell 110 which defines internal dome region 104. The remainder of motor102, including nozzle throat region 106 being defined by athree-directional or four-directional woven carbon-carbon compositeintegral throat entry 120, a unidirectional tape-wrapped carbon phenolicthroat insulator support 122, a metal alloy exit cone shell 124 and acarbon phenolic composite exit cone liner, or insulator, 126 which formsa vectorable-type exit cone assembly, pivotally attached to the aft endof dome shell 110 by pivoting mechanism 128, may be constructedgenerally in the same manner and of the same materials as discussedpreviously with respect to the exemplary prior art motor shown in FIG.2.

[0056] In FIG. 3, shear-ply layer 82 is preferably formed of anelastomeric material containing either silica powder or aramid fibers(e.g., Kevlar® material) and a curable rubber such as ethylene propylenediene monomer (EPDM). This composition is commercially available fromBurke Rubber Company in the form of calendered sheets. Shear-ply layer82 is preferably hand laid into a selected portion of inner surface 81of dome shell 80 by cutting and trimming calendered sheets into theproper size and configuration so as to conform to inner surface 81 ofdome shell 80. A preferred bonding system of Chemlok® 205 primer andChemlok® 236 adhesive available from Lord Corp. is used to ensure aproper bond between the shear-ply layer 82 and the inner surface 81 ofdome shell 10. However, in contrast to the shear-ply layer 12 shown inFIG. 1 which is bonded essentially to the entire or full length of theinner surface of dome shell 10, shear-ply layer 82 covers significantlyless of the inner surface 81 of dome shell 80 and is sized so that itprotrudes only slightly beyond the forward edge of aft insulator 84 tobe bonded within dome shell 80 in due course. Because less surface areaof the inner surface of dome shell 80 is to be covered, considerabletime and expense can be saved with respect to the installation andmachining surface contours of shear-ply layer 82 compared with the morelongitudinally extensive shear-ply layer 12 shown in FIG. 1. Uponshear-ply layer 82 being laid up and trimmed to properly fit thepre-selected area in which it is to be bonded within dome shell 80, theresulting assembly is then preferably vacuum bagged and autoclave cured.

[0057] Aft dome insulator 84 is preformed in much the same way as theaft dome insulator 14 shown in FIG. 1 with respect to construction stepsand preferred material. That is, carbonized fiber tape impregnated withphenol formaldehyde resin, such as Fiberite tape (Fiberite ProductNumber MX 4926) available from the Fiberite Corp., is laid down, on andabout a mandrel in order to construct a preform that is autoclave cured,or optionally hydroclave cured. Upon the preformed insulator 84 beingcured, it is removed from its respective mandrel and machined toproperly fit against and be received by shear-ply layer 82. However,unlike asymmetrical insulator 14 of FIG. 1, insulator 84 shown in FIG. 3is much smaller in surface area and is also symmetrical about itslongitudinal axis. Thus, insulator 84 is much easier to fabricate andtakes less time to construct due to its smaller size and symmetry, plusthere is not nearly as much expensive tape-wrapped material to bescrapped compared to the amount of scrap material generated in machiningthe asymmetrical preformed insulator 14 shown in FIG. 1. This feature ofthe present invention offers yet one more advantage over insulatorsconstructed in accordance with previously known procedures.

[0058] Aft dome insulator 84, upon the outer surface thereof beingproperly machined and prepped, and having its forward and aft edgesmachined and surfaced for interfacing such edges with the aft edge offorward dome insulator 86 and the forward edge of integral throat entry88 respectively, is fitted and bonded against shear-ply layer 82 with anadhesive such as Chemlok® 205 primer and Chemlok® 236 adhesive. Bondinginterface 85, also referred to generally as a secondary bond line, whichin this particular case is the circumferential position in which the aftedge of forward insulator 86 (made of rubber ablative material) abutsforward edge 98 of aft insulator 84 (made of tape-wrapped carbonphenolic composite material), is preferably precoated with an adhesivesuch as EA-934NA or EA-9394available from Hysol-Dexter, Inc.

[0059] Next, calendered sheets, preferably having a width ranging up to66 inches (approximately 168 cm) and a thickness of about {fraction(1/10)}th of an inch (approximately 2.5 mm) and formed from a rubberablative material compound in accordance with the present invention tobe discussed in detail further herein, are laid into dome shell 80preferably after the inner surface of dome shell 80 is precoated with arubber vulcanizing adhesive, such as TY-PLY BN available from the LordCorporation, in order to form forward dome insulator 86. Because aftdome insulator 84 is symmetrically shaped, forward insulator 86 will beasymmetrical about its longitudinal axis. However, lay-up of the rubberablative material is easily accomplished because the sheet material ispreferably laid up directly into dome shell 80, and is not preformedabout a mandrel and subsequently machined to a final contour allowingfor the exit cone to be canted at a preselected angle. This aspect offorming forward dome insulator 86 of a pliable ablative insulativerubber sheet material, instead of a tape-wrapped carbonized fiber resincomposite material which must be preformed, cured, and extensivelymachined, is yet another favorable attribute of the present invention.

[0060] Forward insulator 86 is laid into at least a portion of innersurface 81 of dome shell 80, and preferably covers the remaining exposedportion, or inner surface 83, of shear-ply layer 82, in which shear-plylayer 82 preferably need only extend slightly beyond the forward edge ofinterface secondary bond line 85 to ensure a sound bond at secondarybond line 85. Then, dome shell 80, previously installed shear-ply layer82, and dome insulators 84 and 86, as well as previously installedintegral throat entry 88 and throat insulator support 90, are vacuumbagged as one assembly and preferably cured in an autoclave forapproximately 90 minutes at a temperature of approximately 290° F. (143°C.) and at a pressure of approximately 200 psig (1378 kPa).

[0061] Upon dome insulators 84 and 86 being fully cured and bondedwithin the dome assembly, the internal surfaces of the insulators 84 and86 defining internal dome region 74 can now be machined to a finalcontour and surface finish. Thereafter, the dome assembly can beinstalled onto a motor chamber case (not shown) by way of mountingflange 96 and/or other sub-components of motor 72 can now be secured todome shell 80 such as the exit cone assembly comprising exit cone shell92 and exit cone insulator 94.

[0062] With respect to constructing and installing shear-ply layer 112,aft dome insulator 114, and forward dome insulator 118 of open-endedmotor 102 having a vectorable-type exit cone shown in FIG. 4, thepreferred constructions, steps, and materials employed are as previouslydescribed with respect to constructing such counterpart components ofmotor 72 shown in FIG. 3 resulting in bond lines at surfaces 113, 116,and 119 as shown in FIG. 4. However, it should be appreciated that aftinsulator 114, preferably made of tape-wrapped carbon phenolic compositematerial in essentially the same manner as aft insulator 84, ispreferably symmetrically shaped, instead of being asymmetrically shaped,about its longitudinal axis as is aft insulator 44 depicted in FIG. 2.Furthermore, aft insulator 114 is smaller in size than its prior artcounterpart aft insulator 44, as is shear-ply layer 112 compared to itsprior art counterpart shear-ply layer 42 depicted in FIG. 2. Thus, itshould also be apparent that motor 32 requires three dome insulators,aft insulator 44, middle insulator 46, and forward insulator 48, whereasmotor 102 need only have two insulators, a preferably symmetrical carbonphenolic composite aft insulator 114 and a preferably asymmetric rubberablative material forward insulator 118. By thus eliminating oneinsulator entirely, significant manufacturing and material cost savingscan be enjoyed, as well as the reduction of costs attributable toproperly tracking and maintaining traceability documentation on theeliminated dome insulator. Such cost savings are also provided uponforward insulators 86 and 118 being bonded into inner surface 81 of domeshell 80 and inner surface 111 of dome shell 110, respectively, earlierin the construction process. It is no longer necessary to identify andtrack the preformed insulators and associated forming mandrels of theforward and middle insulators shown in FIGS. 1 and 2 of the drawings.

[0063] Alternatively, forward insulators 86 and 118 as shown in FIGS. 3and 4, respectively, can be precured and machined to final configurationprior to installation within dome shells 80 and 110, respectively. Theforward insulators can be formed to approximately final configuration bymolding under vacuum using processes known within the art. Preferably,an epoxy adhesive such as EA-934NA or EA-9394available fromHysol-Dexter, Pittsburgh, Calif., is then used to bond the precuredinsulator onto shear-ply layers 82 and 112 and/or into dome shells 80and 110, respectively.

[0064] In accordance with the present invention, it is preferred thatforward dome insulators 86 and 118, for example and without limitation,be made from calendered sheet stock comprised of the followingingredients set forth in Table 2: TABLE 2 Ingredient PBW Trade NameVendor Acrylonitrile Butadiene 100 Krynac 40.E65 Bayer Fibers, RubberOrganics, and (65 Mooney viscosity) Rubber Division Akron, Ohio PhenolFormaldehyde 200 Redimix 9821 Harwick Chemical Co. Resin CMS Chemical(120 PBW)/Zinc Borate Division (80 PBW) master batch Wynne, ArkansasZinc Oxide  5 Kadox 930C Zinc Corp. of America (low surface area)Monaca, Alabama Tetramethyl Thiuram  3 Tuex powder Uniroyal Chemical Co.Disulfide Middlebury, Connecticut Stearic Acid  2 Industrene R WitcoCorp. Greenwich, Connecticut Total 310

[0065] Optionally, the phenol formaldehyde resin/zinc borate masterbatch need not initially be prepared as a stoichiometric premixed masterbatch having properly proportioned subingredients of phenol formaldehyderesin and zinc borate, but it is highly preferred for quality controlpurposes and ease of resin dispersion. The preferred subingredients ofthe master batch include phenol formaldehyde resin (120 parts by weightand ground to 100 mesh) marketed under the term “BKR 2620” by GeorgiaPacific Corporation, Decatur, Georgia, and zinc borate (80 parts byweight) marketed under the term “Firebrake ZB” from US Borax Co., LosAngeles, Calif. Other sources may exist and be available as alternatesources for providing some or all of the individual raw ingredientsneeded to form the above compound.

[0066] Generally, the following procedure is used to ensure propermixing, milling, and calendering of the above raw ingredients to formsheets of the subject rubber ablative insulative material.

[0067] Preferably a water-cooled Banbury mixer available from FarrelCompany, Ansonia, Conn., is used to mix the ingredients within apreferred and stable range of temperature. Prior to mixing, the Banburymixer is cleaned with solvent and dusted with a white pigment such ascalcium carbonate to soak up any excess solvent.

[0068] Next the acrylonitrile butadiene rubber (NBR) is added to themixer and mixed for approximately one minute. The master batch,tetramethyl thiuram disulfide, stearic acid, and zinc oxide are thenadded and mixed until the batch temperature reaches approximately 210°F. (99° C.) to approximately 230° F. (110° C.). After being thoroughlymixed to temperature, the batch is then dumped. Verification that thetemperature of the dumped batch is within the above range is achieved byinsertion of a temperature probe in the dumped batch.

[0069] The batch is then placed on a two-roll mill and quickly removedin approximately 3 foot by 4 foot (0.91 m by 1.2 m) pieces and cooled toambient temperature by hanging on a cooling conveyor. The now-coolpieces of material are tested to ensure conformance to specificationrequirements.

[0070] Acceptable material is then placed on a two-roll mill to softenit and then calendered into sheets of about {fraction (1/10)} inch (2.5mm) thickness and preferably ranging in width from about 33 inches to 66inches (83 cm to 168 cm). During the calendering process, the NBR/phenolformaldehyde rubber sheets are provided protection for shipment andstorage by application of a thin sheet of polyethylene film. The film isremoved when the calendered sheets are to be formed and trimmed intoinsulators, or other end products, made of such sheet material of thepresent invention.

[0071] It may be of interest to those skilled in the art that it couldbe necessary to modify the cross-sectional thickness of the insulativematerial of the present invention in comparison to tape-wrapped phenolicmaterials due to heat transfer or char and erosion considerations. Forexample, it can be readily seen that forward insulators 86 and 118respectively illustrated in FIGS. 3 and 4 are significantly thicker incross section than forward insulators 16 and 48 illustrated in FIGS. 1and 2. This consideration should be taken into account when designinginsulators made of materials of the present invention.

[0072] It is believed that the surprising results offered by thematerial of the present invention disclosed in Table 2 compared to theprior art material disclosed in Table 1 is due to the replacement ofboric acid with zinc borate. Testing performed on insulators made inaccordance with the present invention has shown that use of zinc boratein place of boric acid slows the material aging rate. Aging is a problemfor the formed insulator because the aged material becomes stiffer,causing a corresponding reduction in ultimate strain capability, whichaffects how far the material will stretch before it breaks. Insulatorsmade with prior art material tended to crack during motor ignition dueto the reduction in strain capability that occurred during aging. Acrack in the insulator forms a pathway for high-pressure andhigh-temperature combustion gases to exit the motor in an unplannedarea. This condition invariably results in a catastrophic motor failure.The mechanism of aging in the prior art material has been extensivelystudied and has been found to be due to a reaction of boric acid withthe phenyl and methyl alcohol groups found in the phenolic resin. Thisreaction effectively increases the crosslink density of the phenolicresin, which reduces strain capability of the elastomeric ablativematerial. This reaction does not occur with the material of the presentinvention, which replaces boric acid with the nonreactive zinc borate.

[0073] The material of the present invention has the additionaladvantage of producing less water during the cure reaction. The boricacid in the prior art material produces insulation containing about 10%water that can be released during high-temperature cure of theinsulation while zinc borate in the present invention material producesinsulation containing about 5% water. Care must be exercised to ensurethat the water generated is removed from the insulator during cure orsolubilized in the composition due to cure pressure. If the water is notremoved or solubilized, it will expand to form voids in the insulatorwhen the cure pressure is released. Such unwanted voids will act asflaws and negatively affect the structural integrity of the curedinsulator. Solubilized water that functions as a plasticizer in theinsulation will also diffuse out during aging. Prior art insulatorscontaining more water will thus harden and stiffen more during agingthan the insulator of the present invention that contains less water.

[0074] The ablative insulator material of the current invention in itsmost general form may be described as containing a vulcanizable rubbersuch as butadiene acrylonitrile (NBR), a phenolic resin, a flameretardant, and a suitable curing system constituent for the selectedrubber. In addition to the NBR rubber of the preferred embodiment, othervulcanizable rubbers such as butadiene-styrene copolymer (SBR),polychloroprene, polyisoprene, polyurethane, polyepichlorohydrin,ethylene propylene diene monomer (EPDM), polybutadiene, chlorinatedpolyethylene, halobutyl rubbers, and blends of the above polymers wouldalso be suitable for use. Other suitable flame-retardant systems inaddition to zinc borate include alumina trihydrate and antimony oxide incombination with chlorinated hydrocarbons. Depending on the degree offlame retardance desired, a range of 1 to 150 PBW of flame retardant canbe employed in the insulator formulation. The preferred phenolic resinis of the resole type which does not require a hardener to properlycure; however, other resins of the novolac type which require a hardenerto cure can also be employed. For the novolac-type resins, a hardenersuch as hexamethylene tetramine should be included in the formulation ata stoichiometric level for the selected resin. For both types of resins,any of several grades available from different manufacturers can beused. Acceptable formulation levels for the phenolic resin are in therange of 1 to 300 PBW. Optionally, fibers may be included in thematerial formulation of the present invention. Suitable fibers includearamid, cotton (cellulose), sisal, polybenzamidazole, mineral wool,nylon, polyester, or carbon. Acceptable levels for fibers are preferablylimited to a maximum of 40 PBW because their high degree ofreinforcement produces a very stiff and difficult-to-process material.

[0075] The subject rubber ablative insulative material formed of thecompound disclosed above has been described as being particularlysuitable for use in providing insulators that can withstand the hightemperatures, high pressures, and erosive environments in dome regionsof open-ended rocket motors. However, the subject insulative material ofthe present invention is not limited to such a specific application andcan be used to insulate other components and portions of rocket motors.For example, FIG. 5 depicts an igniter assembly 150 having an igniterpellet cup 152 of a rocket motor in which the subject ablativeinsulative material is suitable for use in forming pellet cup 152 whichcontains and insulates the forward closure 154 and the interior of theigniter bottle 156. Igniter pellet cup 152 is formed from patterns cutfrom calendered sheets as described previously and can be laid into amold and cured. Bonded solid propellant igniter grain is designated as158 and igniter BKNO₃ pellets designated as 160.

[0076] Another application particularly suitable for using the ablativeinsulative material of the present invention is for use as a stationarynozzle shell insulator within such a motor as shown in FIG. 6 of thedrawings. Nozzle stationary shell insulator 182 is likewise formed ofsheet material which can either be molded and then bonded to stationaryshell 188 or, optionally, hand laid in place, vacuum bagged, cured andmachined to final contours in the manner discussed above with respect toconstructing, curing and bonding dome insulators 86 and 118 in domeshells 80 and 110, respectively. The open cavity 186 adjacent to thestationary shell 188 and bonded to aft dome insulator premold 190,typically comprising an aramid-filled EPDM material, can experienceenvironments similar to those of the open-ended aft dome geometrypreviously cited. This is particularly true when nozzle exit cone 184 isvectored for a sustained period of time during the firing of the rocketmotor 180. Chamber insulation 192 is typically formed of anaramid-filled EPDM material. Solid propellant grain is designated at194. Furthermore, the use of the ablative material of the presentinvention need not be limited specifically to rocket motors. It can beused in other applications within and outside the aerospace industrywherever there is a need for such a material that can withstand therigors of the high-temperature, high-pressure, and erosive environmentscaused by exposure to combusting fuels.

[0077] Those skilled in the art will understand and appreciate that thepresent invention as defined by the following claims is not to belimited by the particular details set forth in the above-detaileddescription. There are many variations possible without departing fromthe spirit and scope of the claims.

What is claimed is:
 1. A composition for a curable ablative insulativeelastomerized rubber material, comprising: acrylonitrile butadienerubber; zinc borate; phenol formaldehyde resin; zinc oxide; tetramethylthiuram disulfide; and stearic acid.
 2. The composition of claim 1,wherein the maximum parts by weight of each constituent isapproximately: acrylonitrile butadiene rubber—100; zinc borate—80;phenol formaldehyde resin—120; zinc oxide—5; tetramethyl thiuramdisulfide—3; and stearic acid—2.
 3. The composition of claim 2, whereinthe phenol formaldehyde resin is ground and screened to approximately100 mesh, and the ground and screened phenol formaldehyde resin and zincborate ingredients are preweighed and premixed together to form astoichiometric master batch wherein the zinc borate acts as apartitioning agent to inhibit agglomeration of the phenol formaldehyderesin.
 4. A composition for an curable ablative insulative elastomerizedphenolic resin material, comprising: at least one curable rubber; atleast one flame retardant; at least one phenolic resin; and at least onecuring system constituent.
 5. The composition of claim 4, wherein themaximum parts by weight of each constituent of the composition isapproximately: the at least one curable rubber—100; the at least oneflame retardant—80; the at least one phenolic resin—120; and the atleast one curing system constituent—10.
 6. The composition of claim 4,wherein the at least one phenolic resin and the at least one flameretardant constituents are preweighed and premixed together to form astoichiometric master batch wherein the at least one flame retardantacts as a partitioning agent to inhibit agglomeration of the phenolicresin, to which at least one of the other constituents is subsequentlyadded.